Understanding how spatial variation is linked to diversity maintenance in natural communities is a pillar of plant community ecology. Theoretically, a variable landscape can maintain diversity via niche partitioning: different species can trade off in performing better or worse depending on the conditions of the patch they are growing in, and as a result, more species can sustainably coexist in a community than if it were spatially heterogeneous. In the hyperdiverse system of native annual plants in Western Australia, fallen logs may be one of the greatest contributors to generating spatial variation that could help maintain species diversity. Considerable anecdotal evidence suggests that fallen logs generate spatial variation, or patchiness, in the environment (Figure 1), and that species or assemblages of plants may respond differently depending on if they are near logs or not. Despite such anecdotal evidence, it is yet unknown if and how fallen logs contribute to maintaining species diversity in the native annual plant communities of the Western Australian wheat belt.

Figure 1: image of annual plant halos around logs
Figure 1: image of annual plant halos around logs

The project will address the following questions:

Q1) Are/how are plant communities in fallen log patches different from patches that are in the open?

Q2) Why are plant communities in fallen log patches different from patches in the open?

Q3) Are/how are plant species performances affected by proximity to fallen logs?

Hypotheses

The null hypothesis, H0, is that annual plants in fallen log patches are not different in diversity, abundance, or composition from open patches.

In addition to the null hypothesis, the following constitute four, non-mutually exclusive hypotheses concerning how fallen logs may introduce spatial variation in the environment. I include corresponding predictions for how plant communities may differ between fallen log patches as compared to open patches.

H1: Log decomposition creates islands of fertility directly around the fallen log.
Prediction 1: Nutrient composition around logs will be higher than in open plots

Prediction 2: Variation in nutrient composition in log vs open environments will correspond to variation in species composition, abundance, and/or richness in these environments.

Prediction 3: All sown plants will perform best in environments where organic logs have been left ‘insitu’. In locations where logs have been removed or replaced with pvc, the legacy of the nutrient island effect will yield higher sown plant performance than when compared to locations where logs have never been. The effect of the nutrient island in locations where logs have been added to open environments should yeild higher plant performance over time. note: performance is measured in terms of germination rate, survival to fruiting, fecundity, and/or biomass.

H2: Fallen logs alter the microclimate directly around them by providing shade.
Prediction 1: Shade and temperature around logs vs in open plots will be different

Prediction 2: Variation in shade and temperature in log vs open environments will correspond to variation in species composition, abundance, and/or richness in these environments.

Prediction 3: All sown plants will perform best in environments where there are organic or pvc logs, no matter if they have been recently moved or not.

H3: Fallen logs trap dispersing seeds as they are blown along the ground.

Prediction 1: Dispersing seeds accumulate around logs, leading to a denser stand of plants in fallen log patches. Plant abundance in fallen log patches will be higher as compared to open patches. Rare plants will be more common in fallen log patches as compared to open patches

Prediction 2: All sown plants will perform the same in all experimental environments

H4: At least some species perform differently according to variation in log vs. open environments and have short dispersal kernals, causing fitness-density covariance

Figure 2: Photo before germination, after a rain. Notice the seeming wet halo under and around the branch
Figure 2: Photo before germination, after a rain. Notice the seeming wet halo under and around the branch


Experimental Design

In this experiment, 224 plots are arranged in 7 blocks of 32 plots each within the Caron Dam nature reserve. A map can be found here.
note: the location info for 3.02 is probably incorrect as of May 2022, and location info is currently unavailable for plots 6.25 and 7.19

Each block is approximately 30m X 30m in area. Plots are 1m long and linear, and have a pin tag on either end (see Figure 3). The pin tags have the identity of the plot written on them in the form of “blocknumber.plotnumber”. Plots are 1m or more away from each other.

In each block, plot environments can be one of six types:
- A 1m log that is out in the open (open_with_log, 4 plots)
- A 1m log that is a part of a tree (insitu_log, 4 plots)
- A 1m pvc pipe that is out in the open (open_with_pvc, 4 plots)
- A 1m pvc pipe that is a part of a tree (insitu_pvc, 4 plots)
- A plot that is out in the open (open, 8 plots)
- A gap in a log where a log used to be (gap, 8 plots)

In half of the plots (not including open plots), the addition, exchange, or removal of logs or pvc to the environment was implemented in October 2020, before seed dispersal. In the other half of these plots, these manipulations were implemented after seed dispersal, in March 2021.

Within each 1m long plot, there is a ~20cm long microtransect. The ends of the microtransects are marked by a nail and a washer sunken into the ground. Each microtransect is approximately 21 cm in internal length from inner washer edge to inner washer edge. Microtransects are not sided.

In half of all plots, seeds were sown in March 2021 and February 2022. In these plots, 15 seeds each of Trachymene ornata (TROR), Goodenia rosea (GORO), and Trachymene cyanopetala (TRCY) are sown outside of the microtransects as in the diagram. These plants were selected because they represent plants common to communities next to logs (TROR), out in the open (GORO), or both (TRCY). The plots where seeds were sown are called ‘lambda’ plots as noted in Figure 3. In the dataset, the rows with a “1” in the ‘seeding_trt’ column are the plots that had seeds sown into them.

Figure 3: plot schematic
Figure 3: plot schematic

Datasets

The sets of data that we have collected for this arm of the project are the following.

  1. Community data, before and after the experiment was implemented.
    Every year during peak biomass we surveyed plant communities at every centimeter along each microtransect. Plant count and identity information is collected at each centimeter. These data are available from 2020 (before the experiment began), 2021 (one year into the experiment), and 2022 (two years into the experiment).

  3. Performance data of TROR, TRCY, and TROR.

Methods and analysis

Q1 Are/how are plant communities in fallen log patches different from patches that are in the open?
Methods

Analysis
- abundance - diversity (hurdle model) - composition (nmds) - all three years separately

Q2 Why are plant communities in fallen log patches different from patches in the open?
Methods

Analysis
- nutrient analysis (nutrient composition ~ log vs open; abundance ~ nutrient composition , diversity ~ nutrient composition, composition ~ nutrient composition)

Q3 Are/how are plant species performances affected by proximity to fallen logs?
Methods

In the summer of 2021 and 2022, we

Analysis